1. Field of the Invention
This invention relates to planographic printing masters prepared by electrostatographic techniques. More specifically, this invention concerns photoconductive printing masters which are especially suitable for use in direct and offset lithographic printing processes.
2. Description of the Prior Art
Conventional printing processes can generally be classified in the following catagories: (a) relief printing; (b) intaglio printing (c) planographic printing; and (d) porous printing. In relief printing the master or "image carrier" comprises elevated and depressed regions wherein the printed image is prepared by transfer of ink from the elevated areas of the master to the receiving sheet. The reverse situation is found to exist in intaglio printing systems wherein the printing areas of the image carrier are the depressions within the surface of the master and the elevated regions now provide the background for said image. In planographic printing systems, the printing and background regions of the printing master are on substantially the same plane. Porous printing or screen printing processes are distinct from the above systems in two major respects; holes in the screen provide the image areas of the master and the inking of the screen is a part of the image transfer process.
Planographic printing systems are further subdivided into four distinct printing methods. These systems include stone lithography, direct lithography, offset lithography, and collotype. Since the printing and nonprinting areas of the lithographic master are located on substantially the same plane, problems are encountered in lithographic printing which are not experienced in other types of printing systems. The major consideration is the selective inking of the imaged areas of the printing plate while at the same time avoiding the transfer of ink to the nonimaged or background areas of the printing plate. This problem can be overcome by treating the background areas with an ink releasing material. After each successive printing sequence, this ink releasing capability of the non-printing areas must be refreshed by what is commonly referred to in the printing art as "dampening". Dampening comprises the treatment of these nonimaged areas with a so-called "fountain solution" containing essentially water or a mixture of water and alcohol (e.g., monohydric or polyhydric alcohols). Since the printing inks generally used in lithography are greasy, they are repelled by the relatively hydrophilic non-imaged areas whereas they are attracted to the relatively oleophilic imaged areas of the master. It is critical that the nonprinting areas remain receptive to the dampening solution and nonreceptive of the printing ink. It is also equally critical that the printing areas of the master remain receptive to ink and repellent to the dampening solution. With the exception of collotype printing systems all lithographic printing processes require dampening and inking for each printing cycle.
This periodic dampening of the printing master has been known to result in "flow back" of fountain solution into the train of ink rollers during the course of a printing run, thereby, causing emulsification of the ink on these rollers. Another problem inherent in the periodic dampening of the printing master is the "carry out" of fountain solution from such plates to the transfer sheet. Wetting of the transfer sheet with such fountain solutions can cause curling of the sheet, thus, making registration more difficult in the event that such sheet must undergo multiple trips through the press, as is often required in color printing.
The most popular of the planographic printing systems, (based upon commercial considerations), are "direct" and "off-set" lithography. In direct lithography, the ink image is transferred from the printing master directly to a receiving sheet. Offset lithography, on the other hand, involves transfer of the ink image from the lithographic master to a "blanket" or an "impression cylinder" which in turn conveys the ink image to a receiving sheet. While it is apparent that direct lithography provides a simpler printing system due to the absence of an intermediate transfer step, the presence of such a transfer medium does serve a highly desirable function in that it minimizes the "carry out" of a fountain solution from the printing master to the transfer sheet, thus, avoiding wetting of such sheets with fountain solution. The resilient nature of this blanket also enhances transfer image quality and minimizes abrasion of the image pattern on the master.
Until relatively recently, all lithographic printing systems have required some periodic dampening of the printing master between printing cycles in order to renew the ink repellency of the non-printing areas of the lithographic master. A number of lithographic printing systems have been disclosed which reportedly eliminate the need for such periodic dampening. These systems, hereinafter referred to as "waterless lithographic printing systems", base their reported success upon the discovery that certain materials (described as "abhesive" materials) have surface characteristics which defy acceptance of ink from an inking roller and yet are capable of undergoing selective modification (photomechanical or electrical etching) so as to render such modified areas ink receptive, U.S. Pat. No. 3,511,178.
More recently, a waterless lithographic printing system has been disclosed wherein an ink receptive image pattern is formed on an abhesive substrate by conventional electrostatographic imaging methods, German patent application OLS P 22 34 692.5, published Jan. 25, 1973. According to this publication, a photoconductive layer is overcoated with a thin film of abhesive material and the resulting structure charged and imaged in accord with standard electrostatographic techniques. The latent image thus formed on the abhesive surface of the structure is rendered visible by development with toner particles which upon fusion to this abhesive layer provide ink receptive characters on this ink releasing background. Other embodiments described in this publication include preparation of photoconductive binder layers wherein the ink releasing (abhesive) materials serve as a binder for a variety of photoconductive pigments (e.g. zinc oxide or anthracene). All of the resulting lithographic masters prepared in compliance with the teachings of this disclosure are reportedly useful in waterless lithographic printing systems and possess good resistance to image breakdown.
The principal reasons for the reported success of the above systems apparently reside in the surface characteristics of the abhesive material making up the image carrier. The extent to which such image carriers retain their ink releasing properties is dependent upon the maintenance of such favorable surface characteristics. The incorporation of sufficient quantitites of photoconductive pigments within such abhesive materials to render the resulting binder layer photoconductive can, however, result in adverse alteration in the ink releasing properties of such films. This is to be expected since as the concentration of such pigment in the binder increases, the composite layer begins to progressively take on the surface characteristics of the non-abhesive pigments incorporated therein.
It is, therefore, the principal object of this invention to provide a photoconductive lithographic printing master which does not suffer from the deficiencies experienced in such binder layer systems.
Accordingly, it is the object of this invention to provide a photoconductive lithographic printing master suitable for use in both offset and direct lithographic printing systems.
More specifically, it is the principal object of this invention to provide a photoconductive lithographic printing master wherein the ink receptive toner image is firmly adherent to the ink releasing substrate.
It is another object of this invention to provide a photoconductive lithographic printing master which can be prepared by standard electrostatographic techniques.